Secure mechanism for transmitting counter value in the context of transmission of a synchronizing signal in a packet network

ABSTRACT

An apparatus for transmitting and receiving packets in a packet switching network that includes at least two stations. The apparatus includes a phase-locked loop, as well as means for receiving packets containing samples of the network, the samples being derived from data sampled every T ech  period, where T ech  is derived from a time base synchronized on all the stations of the network and the samples being associated with incremental values predicting at least one sample, calculating, in case of interruption of packet reception, a prediction of a sample that should have been received, by adding an increment value to a received sample value.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/FR2007/050919, filed Mar. 13, 2007, whichwas published in accordance with PCT Article 21(2) on Sep. 20, 2007 inFrench and which claims the benefit of French patent application No.0650838, filed Mar. 13, 2006.

SCOPE OF THE INVENTION

The present invention relates to the domain of communication networks.

The present invention relates more particularly to a transmitter deviceand a receiver device for the secure transmission of a synchronisationsignal, for example of “Genlock” type (synchronisation locking), on apacket switching network, of for example IP (Internet Protocol) type.

PRIOR ART

The transmission of a Genlock signal in an IP network is an importantstage in the conversion of video studios to IP networks as a singleinfrastructure.

The principle consists in driving a counter by the gunlock signal thatis to be transmitted. This counter is regularly sampled. The systemperformance is based principally on the regularity of the samplingmechanism. The sampled value is transmitted on the IP network bystandard means. On the reception side, the samples are taken intoaccount regularly (same instants as the sampling on the reception side)by the PLL (“Phase Locked Loop”), which regulates its internal frequencyso that the difference between the received sample and its internalcounter is null.

The prior art discloses, through the European patent application EP 0624 983 (Thomson), a synchronisation arrangement for a compressed videosignal. This European patent application describes a device to developthe synchronisation of an intermediary layer of a signal such as thetransport or multiplex layer of a compressed multilayer video signal.This device comprises, on the encoding side, the means to include atemporal descriptor reference, such as the counter value of a K modulocounter, and the means to provide a differential temporal descriptorthat can be updated by the transit time of the respective circuits(multiplexing) as the signal is in transit in these circuits. At thereception level, a counter reacts according to a clock signal of thecommanded receiver and the value of this counter is sampled upon arrivalof the temporal descriptors contained in the transport layer. Thetemporal descriptors and the differential temporal descriptors areextracted from the signal and combined to form a corrected temporaldescriptor. The successive sampled counter value differences of thereceiver counter are compared with the corresponding successivecorrected temporal descriptor differences in order to supply a signal tocommand the receiver clock signal.

The invention of this European patent application of the prior art iswithin the framework of a broadcast network with a constant transmissionbitrate. The sampling of local counters is carried out at the instant ofreception of packets containing samples of the master counter. Thepurpose of this previous invention is to supply corrective informationenabling re-establishment of a correct sampling value transmitted from amaster counter via a link that functions but that is degraded by atransmission jitter.

SUMMARY OF THE INVENTION

The present invention is situated within the context of a packetswitching network, of the IP type for example, in which the transmissiontime is not constant. The video stream, for which the present inventionproposes to transport the synchronisation signal, is not assumed to beof constant bitrate, or linked to a multiplexing system of informationrelating to the synchronisation signal transmission. It is evenexplicitly assumed that within the context of an IP network, thesynchronisation information uses its own IP packets: two data streams,video and synchronisation, independent from the transmission networkperspective, are present. The present invention assumes that asynchronous signal is available on all the stations, generated byanother time base, synchronised by another independent system, based forexample on the IEC61588 standard, and it is during the occurrence ofthis signal that the samples are generated or taken into account, andnot at the instances of packet reception or transmission that have noimportance in the scope of the present invention.

The technical problem that the present invention proposes to resolveconsists in enabling a receiving station to continue to function, evenin degraded mode, while the link via the network is totally interrupted.

For this purpose, the present invention concerns, according to a firstaspect, a device able to transmit packets in a packet switching network,comprising at least two stations, comprising the means to:

-   -   Means for receiving a signal,    -   Means for driving a counter using said signal, characterized in        that it comprises:    -   Means for sampling said counter at every T_(ech) period, where        T_(ech) is from a time base synchronised on all the stations of        said network,    -   Means for calculating an increment value predicting at least one        next value of the counter sampled,    -   Means for transmitting the sampled values and the increment        values in an associated manner in packets.

According to an embodiment, said signal is of the Genlock type(synchronisation locking).

According to a particular embodiment, the incremental value is 135,000,the sampling period is 5 ms and the counter clock, driven by the Genlocksignal, is 27 MHz.

According to a second aspect, the present invention concerns, a deviceable to receive packets in a packet switching network comprising atleast two stations, characterized in that it comprises:

-   -   a phase locked loop, as well as means for:    -   means for receiving packets containing samples from said        network, said samples coming from data sampled every T_(ech)        period, where T_(ech) is from a time base synchronised on all        the stations of said network and said samples being associated        with incremental values predicting at least one sample,    -   means for calculating, in the case of packet reception        interruption, a prediction of a sample that should have been        received, by adding an increment value to a received sample        value.

Preferably, the time base synchronised on all the network stations is ofthe IEC61588 type.

According to an embodiment, said communication network is of the IP(Internet Protocol) type.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following descriptionof an embodiment of the invention provided purely as an example byreferring to the single FIGURE that shows the transmitter and receiverdevices according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The principle of the genlock signal transmission is based on a regularsampling of the counter. Regular sampling means that the time periodbetween two samples must remain precisely the same. The counter that issampled being assumed to be stable as it is locked on the genlock signalthat is to be transmitted, the increment between the two values sampledmust remain the same, once the system is stabilized.

During the system start-up, the increment value between a sampled valueand the next sampled value can change slightly. This is due to theimprecision of different clocks of the system: the counter clock drivenby the Genlock signal (video counter on the FIGURE) and the system clock(or network clock on the FIGURE) that calculates the sampling timeperiod. For example, if we suppose that the sampling period is 5 ms andthat the counter clock driven by the Genlock is 27 MHz, the incrementvalue should be precisely 135,000. Due to the relative precision of theclocks, the value can be slightly different (135,008 in the case of 30ppm clocks).

The exact increment value can be attained after several samplingperiods. It can also change in time due to the clock frequency drift.The speed is very low for this (typically 75 mHz/s).

The transmission part that is responsible for sampling the counterdriven Genlock signal (video counter on the FIGURE) as well as thetransmission of the sampled value, can also calculate a preciseincrement and transmit it.

It is here assumed that the transmission part transmits an incrementvalue with the sampled value. This increment value is re-calculated witheach sample. This can change to follow the clock drift according to thefrequency characteristics of the genlock signal (maximum drift of 75MHz/s). A more rapid drift can be filtered.

The reception part memorises the increment value every time it receivesa new sampled value, just as it will memorise the sampled value. When anew sampling signal takes place, if no new sampled value has beenreceived (because it was lost in transmission on the IP network), thePLL calculates a prediction of the value that should have been received.It must add the increment value to the last sampled value received.

The PLL can proceed in this way as long as no new sampled value isreceived. It must memorise the calculated sampled value and use it as ifit was the last received value for the next time.

If one or more samples are missing, no difference should exist betweenthe transmitted calculated values and the values that were lost due tothe frequency drift. When the transmission functions again, the valuereceived must be equal to the calculated value. The interruption totransmission should be completely transparent for the PLL. This will bethe case when a packet is lost from time to time in the network. Itshould enable use of a non-secure protocol such as UDP (User DatagramProtocol) to transport the samples and increments.

In the case of a long interruption, the mechanism will ensure that thePLL frequency remains stable and that the re-calculated counter does notdrift too far from the value that it should have had. When thetransmission functions again, there may be a difference between the newsampled value received and the calculated value. The PLL takes intoaccount the new value received and changes so as to compensate for thedifference. The frequency characteristics of the PLL guarantee that thesignal delivered does not drift too rapidly and that it respects theconstraints of the genlock signal.

The single FIGURE shows the transmitter and receiver devices accordingto the present invention.

The invention is described in the preceding text as an example. It isunderstood that those skilled in the art are capable of producingvariants of the invention without leaving the scope of the patent.

1. An apparatus for transmitting packets in a packet switching networkover which an apparatus for receiving said packets is connected, whereinsaid apparatus for transmitting packets comprises: means for receiving asynchronisation signal, means for driving a counter using saidsynchronisation signal, wherein said apparatus further comprises: meansfor sampling said counter at every T_(ech) period, where T_(ech) isderived from a non-transmitted time base synchronised on the apparatusfor receiving connected over the packet switching network at least twostations, means for calculating an increment value predicting at leastone next value of the counter sampled, means for transmitting thesampled values and the increment values in an associated manner inpackets, wherein T_(ech) is derived from the time base independent ofany source content.
 2. The apparatus according to claim 1, wherein thetime base synchronised on all the apparatuses connected over the packetswitching network is of the IEC 61588 type.
 3. The apparatus accordingto claim 2, wherein the synchronisation signal is of the Genlock type(synchronisation locking).
 4. The apparatus according to claim 1,wherein the synchronisation signal is of the Genlock type(synchronisation locking).
 5. The apparatus according to claim 4, thecounter comprising a clock, wherein the incremental value is of 135,000,the sampling period is 5 ms and the counter clock, driven by the Genlocksignal, has a frequency equal to 27 MHz.
 6. The apparatus according toclaim 1, wherein said packet switching network is of the IP (InternetProtocol) type.
 7. An apparatus for receiving packets in a packetswitching network over which an apparatus for transmitting said packetsis connected comprising at least two wherein said apparatus forreceiving packets comprises: a phase locked loop; means for receivingpackets containing samples from said packet switching network, saidsamples coming from data sampled every T_(ech) period, where T_(ech) isderived from a non-transmitted time base synchronised on the apparatusfor transmitting connected over the packet switching network at and saidsamples being associated with incremental values predicting at least onesample, means for calculating, in the case of packet receptioninterruption, a prediction of a sample that should have been received,by adding an increment value to a received sample value wherein Tech isderived from the time base independent of any source content.
 8. Theapparatus according to claim 7, wherein the time base synchronised onall apparatuses connected over the packet switching network is of theIEC 61588 type.
 9. The apparatus according to claim 7, wherein saidpacket switching network is of the IP (Internet Protocol) type.